The BAG-family proteins in Arabidopsis thaliana

The co-chaperone Bcl-2 associated gene product (BAG) proteins modulate Hsp70's activity in animals. There are six BAG genes in human that encode nine isoforms with different subcellular locations. Analysis of the Arabidopsis thaliana genome revealed eight genes encoding proteins with a conserved BAG domain. The BAG-like protein family in A. thaliana can be grouped into three classes based on the presence of other conserved domains. Class I consists of AtBAG-1, AtBAG-2, AtBAG-3, and AtBAG-4, each containing an additional ubiquitin-like domain. These four proteins are structurally similar to the human BAG-1 proteins and might be BAG-1 orthologs in plants. Class II consists of only one protein AtBAG-5 that has no other conserved motifs. Class III includes AtBAG-6, AtBAG-7, and AtBAG-8, which may be regulated by Ca²⁺ because of the presence of a calmodulin-binding domain. "Digital Northern" analysis on transcripts indicates that AtBAG genes fall into three groups based on the significance test: the first group, AtBAG-2, AtBAG-5, AtBAG-6, and AtBAG-8, has less than five expressed sequence tags (ESTs) for each gene; the second group, AtBAG-1, AtBAG-4, and AtBAG-7, has six to seven ESTs; the third group, AtBAG-3, has 17 ESTs. BAG-like genes are also found in other plant species including commercially important crops such as rice, cotton, potato, tomato, and soybean. Other animal Hsp70 co-chaperones such as CHIP (carboxyl terminus of Hsc70-interacting protein) and HOP (Hsp70/Hsp90 organizing protein) are also identified in A. thaliana, indicating that many of their functions are likely conserved in plants as well. A thorough understanding of how co-chaperones function at the molecular level would provide an insight into the mechanisms of Hsp70's regulation by these proteins.